Update to alpine 3.13

core/admin/Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3-alpine3.7
+FROM python:3-alpine3.13
 
 RUN mkdir -p /app
 WORKDIR /app

core/dovecot/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.7
+FROM alpine:3.13
 
 RUN echo "@testing http://nl.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
  && apk add --no-cache \

core/nginx/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.8
+FROM alpine:3.13
 
 RUN apk add --no-cache nginx nginx-mod-mail python py-jinja2 certbot openssl
 # added to fix #522

core/none/Dockerfile

@@ -1,5 +1,5 @@
 # This is an idle image to dynamically replace any component if disabled.
 
-FROM alpine:3.7
+FROM alpine:3.13
 
 CMD sleep 1000000d

core/postfix/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.7
+FROM alpine:3.13
 
 RUN apk add --no-cache postfix postfix-sqlite postfix-pcre rsyslog python py-jinja2